The invention described herein may be manufactured and used by or for the Government of the United States of America for government purposes without the payment of any royalties therefor.
Various systems utilizing laser beams for conveying information require a device for controlling the beam in accordance with the information. A prevalent form of such device is an optical modulator comprised of a semiconductor structure. More particularly, semiconductor p-i-n diodes are utilized to modify the transmission, or reflection, of a laser beam passed through or incident upon the diode.
The i, or intrinsic region of the diode, in one form, includes a plurality of semiconductor layers which define multiple quantum wells distributed between the p and n regions, with the structure imparting certain absorption characteristics to the diode. That is, when a laser beam of a certain wavelength xcex0 impinges upon the diode, a certain, relatively low, absorption is experienced for that wavelength. When a reverse bias is applied to the diode, the absorption characteristic is shifted toward a longer wavelength such that wavelength xcex0 now will experience a greater absorption. This is known as the quantum-confined Stark effect and the difference in absorption without, and with, the reverse bias is then used to convey information.
Existing multiple quantum well semiconductor optical modulators, utilizing, for example, GaAs (gallium arsenide) as the semiconductor, are operated at room temperature and at wavelengths in the range of around 850 nm (nanometers) to 860 nm, relatively near the bandgap energy of the semiconductor. These optical modulators however cannot be effectively used at wavelengths significantly above the GaAs bandgap energy, for example, at wavelengths in the range of around 780 nm to 840 nm. Recently, continuous wave laser diodes in this 780 nm to 840 nm range have become commercially available. Modulators are needed for these lasers for applications related to optical networks and free-space communications for satellites and aircraft. High-power 810 nm lasers have been developed and have been used in prototype laser radar systems (LADAR) and a particular need exists for a room temperature semiconductor optical modulator that works effectively at this, as well as other wavelengths in the 780 nm to 840 nm range. The present invention meets these needs.
A quantum-confined Stark effect semiconductor optical modulator is described, which is operable to modulate light of a particular wavelength in the range of around 780 to 840 nm. The modulator includes a p-i-n diode having p, intrinsic and n regions, as well as first and second electrical contacts for application of a voltage. A modulating voltage source is operable to apply a reverse bias to the first and second contacts. The intrinsic region has a plurality of semiconductor layers defining a plurality of quantum wells having a certain bandgap energy, separated by barrier layers having a certain bandgap energy above that of the quantum wells. The quantum wells include at least two ultra-thin intra-well barrier layers within the quantum well, the ultra-thin barrier layers being of a material having a certain bandgap energy above that of the quantum wells. The width of each ultra-thin intra-well barrier layer is no more than approximately two molecular layers thick. In a preferred embodiment two ultra-thin intra-well barriers are provided thus defining a triple quantum well structure.